Non-executable stack and heap

Starting with version 2.0, NetBSD supports non-executable mappings on
platforms where the hardware allows it. Process stack and heap mappings are
non-executable by default. This makes exploiting potential buffer overflows
harder. NetBSD supports PROT_EXEC permission via mmap() for all platforms
where the hardware differentiates execute access from data access, though
not necessarily with single-page granularity. When the hardware has a
larger granularity, the rule is that if any page in the larger unit is
executable, then the entire larger unit is executable, otherwise the entire
larger unit is not executable.

No compile-time option is needed to enable this software support,
it's always available.

Supported platforms

Here is a list of support levels based on hardware
limitations. In this list "the stack" refers to the traditional UNIX
process stack, not pthreads stacks.

powerpc (e.g., macppc):
The hardware granularity is a segment (256MB) instead of a page.
By default, the segment containing the stack is not executable.

i386:
The hardware support is pretty limited, so the granularity is very coarse.
The current software implementation in NetBSD is that there are only two
regions of the application address space which may be independently
executable or not: the stack and everything else. By default,
the stack is not executable and everything else is executable

Everything else; arm, m68k, mips, sh3,
sparc(sun, sun4c), vax:
No support, the hardware does not differentiate execute vs. data accesses,
so anything readable to a process as data is executable by the process
as code.